The present invention generally relates to an implantable cardiac stimulation device. The present invention more particularly relates to an implantable cardiac stimulation device including a circuit for monitoring impedance of an electrode configuration and providing an impedance digital output proportional to the ratio of a stimulation pulse voltage and current.
Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient""s own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves)and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarization at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
In determining whether all the leads of an implantable cardiac stimulation device are functional, that is, the leads are not shorted or open, such devices may include a lead supervision function block, incorporated into the hardware and/or the software, that determines the resistance between the leads. In the case where a lead is non-functional, the device switches to other leads for sensing and stimulating the heart.
To implement lead supervision function in prior devices, pulse voltage and pulse current are measured substantially simultaneously through a measured data system. The lead impedance is calculated by dividing the measured voltage by the measured current. The division operation is usually done using additional hardware or computer software. Division operations in the software requires significant processing time and power consumption for the pacemaker circuits because it is performed via the pacemaker""s microprocessor. In addition, if the division operation is performed using a dedicated digital hardware, it requires a significant number of logic gates (on the order of a few thousand logic gates) which take up significant chip real estate area. The elimination of either software or a dedicated digital hardware implementation of the division operation is desirable.
The present invention provides a circuit that monitors impedance of an electrode configuration including first and second electrodes in electrical contact with a heart. The circuit is within an implantable cardiac stimulation device which applies stimulation pulses having a current and a voltage magnitude across the electrode configuration.
In accordance with one aspect of the invention, the circuit that monitors impedance of the electrode configuration includes a current sensing circuit that provides a first analog signal representing the magnitude of the current of a stimulation pulse applied to the electrode configuration, a voltage sensing circuit that provides a second analog signal representing the magnitude of the voltage of the stimulation pulse applied to the electrode configuration, and an impedance determining circuit having an analog input coupled to one of the current sensing circuit and the voltage sensing circuit for receiving one of the first and second signals, an analog reference input coupled to the other one of the current sensing circuit and the voltage sensing circuit for receiving the other one of the first and second signals, and a digital output for providing a digital signal proportional to the ratio of the first and second signals.
In accordance with another aspect of the invention, the impedance determining circuit is an analog to digital (A/D) converter. The analog input of the A/D converter is coupled to the current sensing circuit to receive the first signal. The analog reference input of the A/D converter is coupled to the voltage sensing circuit to receive the second signal.
In accordance with another aspect of the invention, the circuit further includes an inverter that generates the reciprocal of the digital output of the analog to digital converter.
In accordance with another aspect of the invention, the current sensing circuit and the voltage sensing circuit include an amplifier.
In accordance with another aspect of the invention, in an implantable cardiac stimulation device, a method of monitoring impedance of an electrode configuration of an implantable cardiac stimulation device, the electrode configuration including first and second electrodes in electrical contact with a heart, the method includes the steps of applying a stimulation pulse to the electrode configuration, the stimulation pulse having a current and voltage magnitude, generating a first analog signal representing the current magnitude of the stimulation pulse applied to the electrode configuration, providing a second analog signal representing the voltage magnitude of the stimulation pulse applied to the electrode configuration, and deriving directly from the first and second analog signals a digital output signal having a value proportional to the ratio of the first analog signal and the second analog signal.
In accordance with a further embodiment of the present invention, the first analog signal may represent the voltage magnitude and the second analog signal may represent the current magnitude applied to the electrode.
In accordance with another aspect of the invention, the deriving step of the method includes applying the first and second analog signals to an analog to digital converter. The deriving step further includes coupling an analog input of the analog to digital converter to receive the first signal, coupling an analog reference input of the analog to digital converter to receive the second signal, and generating the reciprocal of the digital output of the analog to digital converter.
In accordance with another aspect of the invention, the method includes the further step of amplifying the first analog signal prior to the deriving step.
In accordance with another aspect of the invention, the method includes the further step of amplifying the second analog signal prior to the deriving step.